The instant invention relates to a high-pressure gas discharge lamp having an envelope enclosing a discharge space consisting of a translucent densely sintered aluminum oxide, at least one tubular current lead-in member secured in a gas-type manner in the envelope and an end plug having an aperture for the current lead-in member fused to a cylindrical part of the envelope.
In this lamp, the end plug is sealed to the current lead-in member by a glass seal.
Such a lamp generally contains an ionizable filling which frequently is a mixture of mercury, a rare gas and an alkali metal.
A lamp of this type is disclosed in Oomen U.S. Pat. No. 4,721,886. As shown in this patent, a gas-tight seal is provided between the end plug and the current lead-in member by a magnesium oxide containing glass sealing ceramic provided in a capillary space between the end plug and the current lead-in member.
While the magnesium oxide containing glass seal provides a satisfactory gas-tight construction, it has been found that when the lamp is operated so as to provide a high temperature to where the current lead-in member enters through the end plug (for example 750.degree. C. produced during operation of a 150 W lamp), magnesium metal can evaporate from the seal. It is found that in such cases, the magnesium provides a coating on the inner surface of the envelope thereby significantly reducing the lumen output of the lamp. This phenomenon increases with time thereby diminishing the useful life of the lamp.